David Parrish, P.E.
FM Global, Standards Division
Summary: The following article is a part of the National Board Technical Series. This article was originally published in the Winter 2001 National Board BULLETIN. (3 printed pages)
Yankee dryers are very large drying rolls predominantly used in the production of tissue-type paper products.
Considered as parts of papermaking machines, they are critical to an industry which produces about 300 million metric tons of paper each year.
A typical Yankee dryer is a cast-iron cylinder of about 6 m (19.5 ft) outside diameter, 7.75 m (25.5 ft) width, and weighing about 180 t (200 short ton). It is typically heated by 1.1 Mpa (160 psi) steam and operated at 1900 m/min (6200 ft/min) while one or more pressing rolls apply pressure to the paper sheet and dryer felt on the Yankee face. Fuel-fired air impingement drying is typically provided over the Yankee to increase the drying rate and thus the production rate.
A Yankee dryer is an assembly of several large gray cast-iron parts (shell, heads, multi-piece center shaft, and journals). The shell is commonly made to ASME Code Section VIII, Division 2, using SA-278, Class 30, 40, 50, or 60 gray cast-iron material and internally may be of smooth-bore or ribbed construction. Heads, center shafts, and journals are sometimes ductile cast iron or may be fabricated from steel.
The owner/user must maintain control of specific operating parameters of Yankee dryers to address adverse stresses. Control of heating is critical during startup. Yankees have cracked from application of the heated impingement air or from applying steam without first starting rotation. Adverse stresses also develop from application of high impingement heat or full steam pressure before the Yankee is at operating speed and the paper sheet has been applied to the surface. When operating, immediate response is required to reduce heat input if the paper sheet breaks. Controlled cooling is also necessary on shutdown of the machine. Control of water sprays, as might be used for control of temperature at the edge of the Yankee for cleanup or firefighting, is critical to avoid thermal shock.
Yankee dryer steam leaks, vibration, or unusual noise during operation should be investigated promptly and addressed. Observation of an unusual condition requires prompt evaluation. The unusual condition may indicate a critical safety issue or a noncritical production issue. Issues determined to be critical must be corrected immediately. Production issues may be resolved at the next scheduled shutdown. Typically, either deterioration in quality of the paper product or visual steam leaks will indicate a need to repair the shell face of the Yankee.
Yankee dryers should be shut down and inspected annually. Both an internal and external visual inspection are needed. Other inspection methods (dye penetrant, magnetic particle, ultrasonic, acoustic) may also be used. If inspection finding requires repair by grinding, or if thinned areas are found, the remaining thickness should be determined by additional ultrasonic inspection or by another method of thickness measurement. The actual thickness should then be used to determine the concurrent maximum allowable operating steam pressure and maximum press roll load for the planned operating speed.
Repairs and Alterations: Different Types
Repairs made to Yankee dryers are limited to grinding, plastic metal (epoxy) filling, plugging, or thermal spray metal coating. Fusion welding is not an appropriate or allowable repair for cast-iron materials, as the heat input causes cracking.
Yankee dryer-specific repairs include:
- Grinding or machining of the shell’s external surface to remove imperfections caused during the operation.
- Thermal spray coating of external shell to restore surface.
- Plastic (epoxy) metal filling of external pits or imperfections.
- Plugging of external pits or imperfections.
- Sealing through-wall shell leaks by internal application of plastic filler metal and external installation of plug.
- Grinding or machining the heads or internal shell surface to remove cracks or other imperfections.
- Sealing of head-to-shell joint steam leaks with pumped sealant and replacement of head bolts.
- Replacement of internal steam distribution or condensate removal parts.
Yankee dryer-specific alterations may include:
Repair Guide for Yankee Dryers
- Replacement of Yankee dryer pressure-containing parts (e.g., fasteners, heads, center stay) with material other than conforming to that listed on the Manufacturer’s Data Report.
- Grinding or machining of the heads, head-to-shell joint, or shell to a degree that modifies the design described on the Manufacturer’s Data Report.
- Increasing the rotational velocity or press roll loading above limits established by the manufacturer with no change in operating temperature or pressure.
Imperfections, indications, or damage may result from the original casting process, normal operation, or an accident. Areas requiring attention must be analyzed separately to determine cause and appropriate repair or replacement. Examination of the imperfection, indication, or damaged area by one or more nondestructive examination methods may be used to determine the extent of the affected area or volume. Through-wall indications should be examined internally and externally so all leakage paths may be addressed.
General Repair Procedures for Yankee Dryers
Machining of cast iron is a well-known process. Due to the size of the Yankee, machining is typically done with the Yankee still in the paper making machine.
- Grinding or machining of the shell external surface to remove imperfections caused by normal operation: Yankees are originally constructed with excess shell thickness for resurfacing, to restore the finish necessary for production. The Yankee manufacturer provides a chart showing the relationship of shell thickness to operating pressure and press roll loading. When thickness is ground to less than the minimum set by the manufacturer, “derating” the Yankee may be necessary.
- Thermal spray coating of external shell to restore surface: Thermal spray coatings are sometimes used to achieve a particular surface finish or as a wear surface when the shell thickness approaches the minimum specified by the manufacturer. The coating does not contribute to the pressure-containing strength of the shell.
- Plastic metal (epoxy) filling of external pits or imperfections in the shell: Small imperfections in the shell surface may be repaired by plastic metal filling; it cannot be used to restore a pressure boundary (through-shell steam leak).
- Plugging of external pits or imperfections in the shell: External shell surface pits or imperfections may be repaired by driven plug. External plugging is a surface treatment only. The limits for use are:
- cannot be used to restore a pressure boundary (through-shell steam leak).
- drilled hole diameter, depth, and distance between holes not to exceed limits in jurisdictional or construction code.
- plug is the same material as the shell.
- plug is driven-fit, not threaded.
- plugs should be installed to the maximum depth permitted by the code or to at least 20% of the shell thickness.
- plug should have a finished diameter slightly larger than the drilled hole to assure a tight, driven fit.
- plug should be slightly longer than the hole depth.
- plug should be driven, swaged and carefully ground to match the shell surface.
- the location of drilled holes should be recorded on a map of the shell surface.
- Sealing through-wall shell leaks by internal application of plastic filler metal and external installation of plug: Internal pressure boundary repair may be by application of a plastic metal filling material or by threaded plug. Reference the construction or jurisdictional code for plug requirements.
- Grinding or machining the internal shell surface or the heads (internal and external) to remove cracks or other imperfections: Since welding is not an acceptable repair method for cast iron, repair may be accomplished by grinding to remove indications. Structural analysis may be used to confirm that the Yankee has sufficient remaining strength for continued operation at the manufacturer’s allowable limits after the grinding repair is completed.
- Sealing of head-to-shell joint steam leaks: A steam leak at a head bolt may be corrected by first confirming that the bolt is not cracked, then removing and refurbishing the bolt and nut. If the bolt is cracked, replace the nut and bolt. Torque the bolt as specified by the manufacturer.
A steam leak at the head-to-shell joint which cannot be corrected by replacing head bolts may be addressed by pumping a sealant into the joint. Tapped holes in replacement bolts are fitted with grease fittings and a sealant is pumped into the joint or into a sealant-pumping groove provided in the joint.
- Replacement of internal steam distribution or condensate removal parts: Internal parts may loosen or break, causing internal erosion of the shell. These must be replaced or repaired and conform to the Yankee manufacturer’s specifications. Shell thickness should be measured to determine minimum required thickness remaining for planned operating conditions.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.